The present invention relates to a method and apparatus for affixing a cylindrical member in a down-the-hole drill or hammer, often referred to in the industry as a DTH drill. In a one exemplary embodiment, the present invention relates to a method and apparatus for affixing a foot valve assembly in a DTH drill.
FIGS. 1A and 1B illustrate a known percussive drill assembly 1 or DTH drill. FIGS. 1A and 1B are taken, with some edits to the reference numerals for the purposes of this description, from U.S. patent application Ser. No. 12/366,014 filed Feb. 5, 2009, the entire contents of which are incorporated by reference into the present application. These prior art figures are provided to illustrate one assembly of a known DTH drill. There are many variations on assemblies of DTH drill but FIGS. 1A and 1B illustrate some of the basic components for background purposes.
With reference to FIGS. 1A and 1B, the known DTH drill 1 includes a casing 2 with lower and upper ends 2a, 2b, and a top sub 3 interconnected with the upper end 2b of the casing 2. The casing 2 includes a central bore 4, a central axis AC extending through the bore 4 between the two ends 2a, 2b, and a fluid supply chamber 5 defined within the bore 4. A source of motive fluid connects to the top sub 3 for the supply of motive fluid to the fluid supply chamber 5. The DTH drill also includes a bit 6 at the lower end 2a of the casing 2, and a shank 6′ that extends up into the casing 2. In other embodiments, the bit 6 and shank 6′ are formed integrally with each other, and throughout this specification, the term “bit” is intended to include the bit 6 and shank 6′ whether or not the two parts are formed integrally or separately. The bit 6 includes an upper end 6a, a lower end 6b, and a bit bore 6c extending from the lower end 6b through the bit 6 along the central axis Ac.
FIG. 1C further illustrates a representative lower portion of a prior art DTH drill, including more details of a representative bit 6. The bit bore 6c has a bit bore diameter 6d. A counter bore 6e is formed in the upper end 6a of the bit 6 coaxially with the bit bore 6c (i.e., both the bit bore 6c and the counter bore 6e are centered on the central axis Ac), and has a counter bore diameter 6f larger than the bit bore diameter 6d. The bottom of the counter bore 6e defines a ring-shaped shoulder 6g extending from the counter bore wall 6e to the top of the bit bore 6c. The counter bore 6e includes cavities 6h in the counter bore wall 6e. The cavities 6h can be a continuous slot extending around the circumference of the counter bore wall 6e or may include one or more discrete, unconnected cavities spaced around the circumference. In some embodiments of the prior art, the counter bore 6e is cylindrical with side walls parallel to the central axis Ac. In other prior art embodiments, the counter bore 6e has tapered walls that are not parallel to the central axis Ac such that the counter bore 6e widens or narrows from the upper end 6a to the bottom of the counter bore 6g. Although embodiments of the present invention are described and illustrated with counter bores 6e that are cylindrical, the present invention can be applied to all types of counter bores 6e. 
Referring again to FIGS. 1A and 1B, a piston 7 is movably disposed within the casing bore 4. The piston 7 includes an upper end 7a, a lower end 7b, and a piston bore 7c extending through the piston 7 between the upper end 7a and the lower end 7b. A valve member 8 is movably disposed within the casing bore 4 generally between the piston 7 and the casing upper end 2b. A distributor cylinder 10 is disposed within the casing bore 4 and receives the valve member 8 in an upper end 10a, and receives the upper end 7a of the piston 7 in a lower end 10b. The valve member 8 and distributor cylinder 10 together regulate and direct the flow of motive fluid from the supply chamber 5 to a drive chamber 12a above the piston 7 and a return chamber 12b below the piston 7. The cyclical provision of motive fluid to the drive and return chambers 12a, 12b causes the piston 7 to rise and lower within the casing bore 4. The cyclical up and down motion of the piston 7 causes the bottom end 7b of the piston 7 to cyclically strike the upper end 6a of the bit 6 to drive the drilling operation of the bit 6.
An exhaust tube 14 is mounted within the counter bore 6e and extends out the upper end 6a of the bit 6 into the return chamber 12b. As the piston 7 approaches the lower position (FIG. 1A), the exhaust tube 14 extends into the piston bore 7c, which cuts off communication between the bit bore 6c and the return chamber 12b through the exhaust tube 14. As the piston 7 approaches the raised position (FIG. 1B), the exhaust tube 14 is removed from the piston bore 7c and communication between the bit bore 6c and the return chamber 12b is established through the exhaust tube 14.
Referring once again to FIG. 1C, the exhaust tube 14 is inserted into the counter bore 6e. In the illustrated construction, the exhaust tube 14 is a relatively simple plastic tube that includes humps 14a on the outer surface of its lower portion. In some embodiments, the humps 14a can extend all the way around the tube 14 like a ridge, and in other embodiments the humps 14a could be one or more bulges in the side of the tube 14, corresponding to one or more of the cavities 6h discussed above with respect to the counter bore 6e. The exhaust tube 14 is inserted into the counter bore 6e until the distal end of the exhaust tube 14 abuts the shoulder 6g at the bottom of the counter bore 6e. When the exhaust tube 14 is so inserted, the humps 14a align with and are received within the cavities 6h. Although in the illustrated embodiment the humps 14a do not extend to the back of the cavities 6h, in other embodiments the humps 14a do extend to the backs of the cavities 6h and the interaction between the humps 14a and the cavities 6h gives rise to a large portion of the interference load. In other embodiments, there may not be cavities 6h, but the tube may include the humps 14a which bear directly against a wall of the bit bore 6c or counter bore 6e and the assembly may rely substantially entirely on the frictional interface between the humps 14a and the side wall of the bit bore 6c or counter bore 6e for the interference fit. The exhaust tube 14 in a DTH drill bit provides a vital function for the drill to operate continuously. In early days of DTH drill design, aluminum tubes were used. Due to the high material cost and complicacy of installation, aluminum tubes were replaced by plastic tubes in the 1980's. One popular plastic material for such tubes is Delrin® plastic. Delrin® is a registered trademark of E. I. Du Pont de Nemours and Company.
The exhaust tube 14 is held in the bit counter bore 6e by way of an interference fit and also by virtue of the interaction of the humps 14a and cavities 6h. The undeformed original outer diameter of the exhaust tube 14 is slightly larger than the diameter 6f of the counter bore 6e. The exhaust tube 14 is forced into the counter bore 6e, which causes the exhaust tube 14 to deform. The shape memory of the plastic material in the exhaust tube 14 causes the exhaust tube 14 to expand against the wall of the counter bore 6e. This gives rise to a gripping force between the exhaust tube 14 and the wall of the counter bore 6e. The gripping force is a function of (e.g., proportional to) the pressure exerted by the exhaust tube 14 against the wall of the counter bore 6e, and also the surface area of contact between the exhaust tube 14 and the wall of the counter bore 6e. 
One aspect of the present invention is recognition that while plastic exhaust tubes offer many advantages, a persistent problem has been that such exhaust tubes tend to loosen during their service life when compared to exhaust tubes made of aluminum, steel, or other suitable materials (collectively, “other suitable materials”). The embodiments of the present invention described below are primarily focused on an apparatus and method for securing a plastic exhaust tube in counter bore of a bit to reduce the likelihood that the exhaust tube will come loose during ordinary operation of the DTH drill. The invention is applicable, however, to securing substantially any cylindrical members within a component of a DTH drill in a bore or counter bore in the component.